1. Field of the Invention
This invention relates to electrode materials for electrochemical capacitors. More particularly, the invention relates to such electrodes comprised of proton inserted ruthenium oxide, or other metal oxides or mixed metal oxides which have the maximum energy density.
2. Prior Art
Electrochemical capacitors (EC's) are devices which store electrical energy at the interface between an ionically-conducting electrolyte phase and an electronically conducting electrode material. EC's were first described in a 1957 patent by Becker, U.S. Pat. No. 2,800,616 issued in 1957. The first practical devices were pioneered by SOHIO as described in U.S. Pat. No. 3,536,963 based on the double-layer capacitance developed at the interface between high-area carbon electrodes and sulfuric acid electrolyte solution. A complementary system, but originating from a different electrochemical phenomenon, that is development of pseudocapacitance associated with a surface reaction, was developed by Conway in 1975, in collaboration with Continental Group, Inc. See Can. Pat. No. 1,196,683 issued in 1985 to Craig. The materials possessing pseudocapacitance discovered in Conway et al.'s work are metal oxides which include ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), cobalt oxide (CoO.sub.2), molybdenum oxide (MoO.sub.2), and tungsten oxide (WO.sub.3). See Conway, Journal of the Electrochemical Society, vol.138, pp. 1539-15, 1991. The most effective material discovered was RuO.sub.2 which gives a reversibly accessible pseudocapacitance of many Farads per gram over a 1.4 V range.
Heretofore, RuO.sub.2, has been fabricated by the thermal decomposition of ruthenium chloride or hydrous ruthenium chloride. RuO.sub.2, like other dioxides of the platinum group, e.g. RhO.sub.2, OSO.sub.2, and IrO.sub.2, exhibits metallic conductivity and possesses the rutile structure. The pseudocapacitance, which arises at the RuO.sub.2 and the electrolyte interface, is due to the facile ionic species absorption on the surface of the RuO.sub.2 electrode material.
In order to maximize the charge or energy storage per unit weight of oxides in this type of system, it is desirable to maximize the surface area of the electrode material. Such a maximum BET surface area of 130 m.sup.2 /gram was achieved by Raistrick for optimized processing. See Raistrick, Proceedings of First Conference on Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 9-11, 1991, Ansum Enterprises Inc., Boca Raton, Fla. The observed capacitance per unit mass (F/g) and the observed capacitance per unit area (F/cm.sup.2), which are determined from the measured electrochemical capacitance, the measured surface area, and the known amount of RuO.sub.2 present in the electrode, are 380 F/g and 200-300 mF/cm.sup.2, respectively, in a 1 V range in sulfuric acid electrolyte. Based on the assumption that one H may be adsorbed on each exposed O atom, a charge density of 200 mC/cm.sup.2 is estimated by Raistrick. This suggests that the observed capacitance 380 F/g is the maximum value that can be achieved for RuO.sub.2.
During charging of the capacitor, a large number of protons from the electrolyte will react with the RuO.sub.2.xH.sub.2 O electrode (see Eqs. (1) to (4)) infra. The use of either the RuO.sub.2 film electrodes as described in the Canadian Patent by Craig or the amorphous phase of RuO.sub.2.xH.sub.2 O in an electrochemical capacitor will result in a gas production and a loss of water in initial cycles. This is due to the fact that either RuO.sub.2 film or RuO.sub.2.xH.sub.2 O has an open circuit potential about 1.0 V vs. the saturated calomel electrode (1.24 V vs. standard hydrogen electrode) in a 5.26 mol sulfuric solution. When the capacitor is first charged, protons will be inserted in the negative electrode. At the positive electrode, the further oxidation of ruthenium results in the decomposition of water since the potential of ruthenium oxide is about the potential for the decomposition of water. The reactions at each electrode and the overall reaction can be described as follows: ##STR1##
The evolution of oxygen gas at the positive electrode will permeate through the electrolyte and accumulate at the separator. The oxygen evolution causes the increase of the resistance because of the gas accumulation inside the capacitor, reduces the cycle life, and increases the leakage current of the capacitor as described by Conway in 1995. The consumption of water will result in an increase of electrolyte concentration, and therefore an increase of resistivity of the electrolyte. The change of the ion concentration in the electrolyte might cause the increase of the resistance of the capacitor and the reduction of the operational temperature range. Part of the energy used in charging the capacitor was used to decompose water, therefore the energy efficiency is low in the first cycle. Higher efficiency is achieved in later cycles. However, the damage to the cell has already been done. Therefore, it is desirable to eliminate the gas evolution and the increase of resistivity after the initial cycles. The present invention addresses this need.